Certain radiation sensors, bolometers, used in space research demand a temperature of less than 1 Kelvin, if good energy resolution is wanted. The temperature range 100-300 mK cannot be achieved using simple devices. A temperature of about 0.3-1 K can be achieved using a Helium-3 (He.sub.3) evaporator, but even this is insufficient in certain cases.
A Peltier refrigerator utilizing a superconductor element is described in the paper Nahum, M. et al; Applied Physics Letters 65, 1994, page 31-23. Normal metal/insulator/superconductor, i.e. NIS junctions operate at extremely low temperatures of less than 10 Kelvin. An NIS refrigerator has the drawback that it is difficult to thermally insulate the cooled normal metal element. An improved refrigerator construction is described in the paper Leivo, M. M.; Pekola, J. P.; Averin, D. V. `Efficient Peltier refrigeration by a pair of normal metal/insulator/superconductor junctions`, Applied Physics Letters 68 (14), 1996, page 1996. When two NIS junctions are connected in a SINIS structure, the connected normal metal element is thermally insulated on both sides.
In addition to the cooling element, the object itself that is to be cooled must be thermally insulated from the substrate. On the other hand, the heat transmitted to the superconductor in the NIS junctions must be effectively conducted away, so that it does not limit the cooling power. In known solutions, the cooling has only been a few milliKelvins at 300 mK, though in theory a considerably greater temperature difference could have been achieved.